The demands of the construction industry for cranes of ever-increasing load lifting capacity have taxed the capabilities of crane manufacturers to satisfy these demands with existing materials to the utmost.
Recently, crane booms of increased efficiency and lifting capacity have been created including telescoping booms which are trapezoidal in cross section, and also possessing the cross sectional form of an inverted trapezoid or A-frame. While these efficient geometric shapes have resulted in a significant lifting strength to boom weight ratio increase, nevertheless the need for still greater lifting capacity exists, preferably without significantly increasing the size and weight of the boom itself, which obviously can defeat the purpose or objective of the construction.
It has been proposed in the prior art to provide openings in the side walls of beams, girders and crane booms for the overall purpose of reducing their weights while maintaining sufficient load-bearing capacity. However, in connection with the larger types of construction cranes now being employed, the weight advantage gain by the formation of openings in boom section side walls can be more than offset by the resulting loss in resistance to buckling of the side walls under loading, so that the net result can be a loss in the load lifting capacity of the crane boom, unless the side walls or plates are increased in thickness or otherwise braced which, again, can defeat the weight saving purpose of the side wall openings. As a consequence, the construction crane industry continues to seek solutions to the continuing problem caused by the ever increasing demand for crane booms of greater load lifting ability.
The present invention seeks to provide a solution to this problem or need in the art, particularly in connection with one recent form of boom or boom section, as disclosed in U.S. Pat. No. 3,807,108, issued Apr. 30, 1974. This patent discloses a hydraulically operated telescoping crane boom whose interfitting boom sections have the cross sectional shape of an inverted trapezoid or A-frame. The patent discloses for at least some of the boom sections continuous longitudinal side plate stiffeners in the form of vertical plate elements having their upper and lower longitudinal edges attached, respectively, to the boom section side plates and bottom plates adjacent the side edges of the latter. The stiffeners are of much lesser depth than the main side plates of the boom section and their purpose is to increase the buckling strength of the side plates significantly.
The present invention or improvements take advantage of the construction disclosed in the aforementioned patent in such a way as to render it possible and practical to provide large side plate openings along the length of a boom section or sections without increasing the thickness of the side plates and in many instances enabling the side plate thickness to be reduced and without reducing their resistance to buckling under load. Therefore, the invention has for its main object to improve upon the boom or boom section disclosed in U.S. Pat. No. 3,807,108 by the use of additional means having a direct cooperative relationship with the side wall stiffening means shown in the patent.
More particularly, in the present invention, an inverted trapezoidal boom section having the continuous longitudinal side wall stiffeners or reinforcing plates disclosed in said patent is additionally provided with a series of side plate or side wall apertures, whose presence eliminates large sections of metal from the side plates and thus greatly reduces the weight of the boom section. Reinforcing stress transmitting rings are positioned in the side plate openings so as to project inwardly and outwardly of the side plates, and the bottoms of these rings are positioned in stress transferring relationship with both the boom section side plates and the longitudinal vertical stiffeners for the side plates which are also joined to the boom section bottom plate at the longitudinal edges thereof. As a result, stresses which tend to buckle the side plates of the boom section and which are normally transmitted diagonally from top to bottom are now enabled to be transmitted around the side plate opening reinforcing rings to the mentioned side plate stiffeners and adjacent lower portions of the side plates. These coacting elements provide the effect of multiple columns along the opposite side walls of the boom section providing great resistance to buckling. Cooperating top to bottom brace bars on the boom section side plates between the several openings and reinforcing rings further resist buckling and these brace bars also have their lower ends attached to the longitudinal stiffeners for the side plates immediately below said rings. The total result of the invention is a significant reduction in the weight of the boom or boom section for a given size and load lifting capacity. Therefore, it becomes feasible and practical with existing materials to meet the needs of the industry. More particularly, by means of the invention, the side plates of an inverted trapezoidal boom section may be materially reduced in weight while maintaining adequate resistance to buckling under loads of a magnitude which it is required to lift in a given crane boom size or design.
Other features and advantages of the invention will become apparent during the course of the following description.